550 
 WTNT44 KNHC 092030
 TCDAT4
 HURRICANE IVAN DISCUSSION NUMBER  30
 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
 5 PM EDT THU SEP 09 2004
  
 BOTH NOAA AND AIR FORCE RECONNAISSANCE PLANES HAVE BEEN IN THE EYE
 OF IVAN DURING THE PAST FEW HOURS. MINIMUM PRESSURE REMAINS AT
 ABOUT 921 MB BUT MAXIMUM FLIGHT LEVEL WINDS HAVE DECREASED TO 144
 KNOTS.  THIS SUGGESTS THAT INITIAL INTENSITY CAN BE LOWERED TO 130
 KNOTS AT THIS TIME.  THERE WERE SOME INDICATIONS FROM MICROWAVE
 DATA AND NOW CONFIRMED WITH THE PLANE THAT IVAN HAS A DOUBLE
 EYEWALL STRUCTURE. THIS MAY EXPLAIN THE RECENT SLIGHT WEAKENING.
 THE HURRICANE WILL PROBABLY FLUCTUATE BETWEEN CAT 4 AND 5 WHILE
 MOVING THROUGH AN AREA OF LOW SHEAR AND HIGH OCEANIC HEAT CONTENT
 BEFORE REACHING CUBA. THEREAFTER...THE EFFECTS OF LAND AND
 INCREASING SHEAR WILL CAUSE A GRADUAL WEAKENING.
  
 IVAN CONTINUES TO MOVE TOWARD THE WEST-NORTHWEST OR 300 DEGREES AT
 13 KNOTS.  THIS GENERAL MOTION WITH A GRADUAL DECREASE IN FORWARD
 SPEED IS EXPECTED DURING THE NEXT 2 TO 3 DAYS.  AS MENTIONED IN THE
 PREVIOUS DISCUSSION...THERE IS HIGH CONFIDENCE IN THIS PORTION OF
 THE FORECAST BECAUSE THE HURRICANE WILL BE STEERED BY THE FLOW
 SURROUNDING A WELL-ESTABLISHED SUBTROPICAL RIDGE. ALL MODELS ARE
 HIGHLY CLUSTERED DURING THIS PERIOD...BRINGING THE HURRICANE
 NEAR OR OVER JAMAICA...THE CAYMAN ISLANDS AND WESTERN CUBA WITH A
 DECREASE IN FORWARD SPEED. THEREAFTER...THE TRACK CONTINUES TO BE
 UNCERTAIN SINCE STEERING CURRENTS ARE FORECAST TO WEAKEN AND MODELS
 DIVERGE. HOWEVER...MODELS FROM 12Z ARE IN SLIGHTLY BETTER AGREEMENT
 THAN IN EARLIER RUNS...AND IN GENERAL HAVE SHIFTED A LITTLE BIT
 WESTWARD. THE OFFICIAL FORECAST ALSO SHOWS A SLIGHT WESTWARD SHIFT.
  
 THE WIND RADII WERE ADJUSTED BASED ON THE STEPPED FREQUENCY
 MICROWAVE RADIOMETER DATA ONBOARD THE NOAA P3 PLANE CURRENTLY
 INVESTIGATING IVAN.
  
 FORECASTER AVILA
  
 FORECAST POSITIONS AND MAX WINDS
  
 INITIAL      09/2100Z 15.0N  72.5W   130 KT
  12HR VT     10/0600Z 16.1N  74.2W   130 KT
  24HR VT     10/1800Z 17.5N  76.2W   135 KT
  36HR VT     11/0600Z 18.8N  78.0W   130 KT
  48HR VT     11/1800Z 19.7N  79.3W   140 KT
  72HR VT     12/1800Z 21.5N  81.5W   140 KT
  96HR VT     13/1800Z 25.0N  83.0W   115 KT
 120HR VT     14/1800Z 28.0N  83.5W   100 KT
  
  
 $$